Characterizing the effects of 5-HT2C receptor ligands on motor activity and feeding behaviour in 5-HT2C receptor knockout mice
Introduction
The 5-HT2 receptor family comprises the 5-HT2A, 5-HT2B and 5-HT2C receptor subtypes, each sharing approximately 46–50% sequence homology (Barnes and Sharp, 1999, Hoyer et al., 2002). Various agonists for these receptors have been identified. Early examples of 5-HT2C receptor agonists include the piperazine analogs mCPP (Curzon and Kennett, 1990) and MK-212 (Clineschmidt et al., 1977). However, these compounds, particularly mCPP, have negligible selectivity for the 5-HT2C receptor over other 5-HT subtypes (e.g. Hamik and Peroutka, 1989, Porter et al., 1999), requiring antagonist studies to fully interpret receptor activity. Two 5-HT2C receptor antagonists are SDZ SER 082 (Nozulak et al., 1995) and SB242084 (Bromidge et al, 1997; Kennett et al., 1997). Next, Ro 60-0175 emerged as a 5-HT2C agonist, albeit with significant agonist property at the 5-HT2B receptor (Martin et al., 1998, Porter et al., 1999). Nonetheless, as with mCPP and MK-212 a significant body of data derived partly from studies with 5-HT2C receptor antagonists supports the value of this compound as a 5-HT2C receptor agonist (e.g. Clifton et al., 2000, Higgins and Fletcher, 2003).
Driven by the therapeutic potential of 5-HT2C receptor agonists, newer compounds have been developed including WAY161503 (Rosenzweig-Lipson et al., 2006), lorcaserin (APD356) (Smith et al., 2005, Thomsen et al., 2008) and CP809,101 (Siuciak et al., 2007). Pharmacological data suggest that these compounds, compared to mCPP, MK212 and Ro 60-0175, have greater selectivity for the 5-HT2C receptor (see Table 1), which is important given the adverse properties associated with activation of 5-HT2A and 5-HT2B receptor subtypes (Barnes and Sharp, 1999, Rothman et al., 2000). Preclinical studies with these drugs suggest that selective 5-HT2C receptor agonists have anti-psychotic, anti-depressant and anti-obesity potential (Cryan and Lucki, 2000, Jones and Blackburn, 2002, Nilsson, 2006, Siuciak et al., 2007), and are effective in reducing drug self-administration and reinstatement (Higgins and Fletcher, 2003, Bubar and Cunningham, 2008).
The 5-HT2C receptor knockout mouse provides a further tool to explore the function of this receptor subtype (Tecott et al., 1995), including a means for determining drug specificity at this target. In wild-type animals mCPP reduces locomotor activity, but in 5-HT2C receptor knockout mice mCPP paradoxically elicits hyperactivity. Subsequent pharmacological studies have shown that this effect results from 5-HT1B/2A receptor activation by mCPP (Heisler and Tecott, 2000, Dalton et al., 2004). Thus, removal of the 5-HT2C receptor results in the unmasking, or expression, of additional pharmacological properties of mCPP.
The present studies examined the effects of a number of 5-HT2C receptor agonists (mCPP, Ro 60-0175, MK212, lorcaserin, CP809,101 and WAY161503) on motor activity in 5-HT2C receptor knockout mice to determine their in vivo functional selectivity for the 5-HT2C receptor. The effects of dexfenfluramine were also examined because many of its in vivo effects appear to be mediated via 5-HT2C receptors, despite the fact that it is a general 5-HT releaser (Fuller et al., 1988). For example, the anorectic effect of dexfenfluramine is attenuated in 5-HT2C receptor knockout mice (Vickers et al., 1999). In addition, because there are occasional reports suggesting that 5-HT2C receptor antagonists increase locomotion (Martin et al., 2002), we included SB242084 and SDZ SER 082 in these studies. Based on the results of these studies we selected several ligands to compare the profile of effects on locomotor activity with their effects on food intake in a palatability-induced feeding test. These drugs were the 5-HT2C receptor agonists mCPP, Ro 60-0175 and lorcaserin and the 5-HT2C receptor antagonist SB242084. The feeding study with lorcaserin is of particular interest because this drug is presently in a Phase 3 obesity trial, having successfully demonstrated efficacy in a 12 week Phase 2 trial (Smith et al., 2009).
Section snippets
Animals and housing
Adult 5-HT2C knockout and wild-type mice (8–12 weeks of age at the beginning of experiments) were used. For all studies involving knockout mice animals were obtained from our in-house breeding colony. Mice were group housed, with ad libitum access to food and water under a 12 h light/dark cycle (lights on at 8 am). For one pharmacological study examining the interaction between SDZ SER 082 and Ro 60-0175 male C57BL/6 mice were purchased from Charles River (St. Constant, Quebec). The 5-HT2C
Comparison of basal motor activity counts, and food intake, following vehicle treatment in wild-type and 5-HT2C receptor knockout mice
Basal activity counts between experiments for wild-type mice injected with saline appeared to vary across experiments. A one-way analysis of variance was used to analyse these counts across the various experiments. This confirmed a significant main effect of group (F8,56 = 3.26, p < 0.001). A Bonferroni test, to allow for all possible group mean comparisons, showed that only the MK212 and SB242084 groups were significantly different from each other. Indeed, following a one-way analysis of
Discussion
There is a considerable body of experimental data describing hypolocomotor effects of various 5-HT2C receptor agonists in rodents. Such effects have been described for mCPP in rats and mice (Kennett and Curzon, 1988a, Kennett and Curzon, 1988b, Lucki et al., 1989, Gleason et al., 2001), MK-212 (Lucki et al., 1989), Ro 60-0175 (Kennett et al., 2000, Higgins et al., 2001), WAY161503 (Mosher et al., 2005), and CP809,101 (Siuciak et al., 2007) in rats. 5-HT2C receptors are localized on inhibitory
Acknowledgements
This work was supported in part by an operating grant from the Canadian Institutes of Health Research to PJF. Some of these data were presented at the 37th Annual Meeting, Society for Neuroscience, San Diego, CA. November 2007.
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